At some point that’s true, but don’t they run the n-1 or 2 generation production lines for years after the next generation launches? There’s a significant capital investment there and my understanding is that the cost goes down significantly over the lifetime as they dial in the process so even though the price is lower it’s still profitable.

Unless plans have changed, the foundries making DDR4 are winding down, with the last shipments going out as we speak.

This is only true as long as there's enough of a market left. You tend to end up with oversupply and excessive inventory during the transition, and that pushes profit margins negative and removes all the supply pretty quickly.